The present invention relates to a high intensity discharge (HID) lamp, of the type having a ceramic metal halide arc tube having opposed axial leads connected to current carrying support members embedded in a glass stem, and a glass envelope having a closed end opposite from the stem.
FIG. 1 shows a known ceramic metal halide discharge lamp having a metal base 10 and a center contact 12 soldered to respective current inputs 16, 17 embedded in a glass stem 14, where they are welded to support members 60, 61. The first support member 60 has an upper end provided with a loop 64 positioned around a protrusion formed by dimple 24 in closed end 22 of outer envelope 20, and a lateral arm 62 welded to a first axial lead 38 of arc tube 30. This is known as an elliptical dimple (ED-type) lamp. The second support member 61 is connected to the second axial lead 39 of arc tube 30 by means of a flexible niobium C hook 66 welded in place. The hook relieves stress that would otherwise be experienced by the arc tube due to expansion and contraction along its axis. The extended sleeves 36, 37 of the arc tube are particularly sensitive to shearing stresses (torque perpendicular to the long axis of the arc tube). However the hook has an inherent material cost as well as the cost of labor to weld it in place. Further, both the arm 62 and the hook 66 can bend if the lamp is dropped, putting the arc tube 30 off center in the envelope, and likewise off center with respect to a reflector in a lighting fixture. This is especially troublesome with a parabolic reflector, which requires that the light source be at the focal point of the parabola.
U.S. Pat. No. 3,906,275 discloses an ED-type lamp with a quartz metal-halide discharge tube held by straps clamping the opposed pinches and fixed to respective mounting members embedded in the stem. Thermal stresses are not a problem for this type of lamp due to the robust character of the quartz envelope, and the relatively low coefficient of thermal expansion of quartz, 5.5.times.10.sup.-7 per .degree. C.
U.S. Pat. No. 4,401,913 discloses high pressure sodium ED-type lamp having a polycrystalline alumina (PCA) ceramic arc tube. The lower terminal is secured to a rod in the stem, and the upper terminal is secured to a rod having an upper end with a helical spring engaging the dimple. The spring compresses to absorb linear expansion of the arc tube, which can be significant due to its length (11 cm), its coefficient expansion (80.times.10.sup.-7 per .degree. C.), and high temperatures. Electrical current is provided to the upper terminal by a fine helically bent wire, known as a flying lead, which does not provide any support. The upper end of the arc tube is supported exclusively by the spring engaging the dimple, so that expansion relative to a support frame is not a problem.